CN105992439A - Linear light emitting diode driver and control method thereof - Google Patents

Abstract

The invention provides a linear light-emitting diode driver and a control method thereof. The linear light emitting diode driver also comprises a protection circuit which judges whether the transient high voltage change occurs or not according to at least one of the voltage of the control end of the transistor and the voltage of the output end of the transistor and triggers the protection function.

Description

Linear light-emitting diode driver and its control method

technical field

The invention relates to a linear light-emitting diode (LED) driver, in particular to a linear LED driver and a control method thereof which can prevent circuit abnormality or instability caused by excessive transient voltage changes.

Background technique

The current LED drivers can be divided into isolated and non-isolated. The isolated LED driver needs a transformer to separate the primary side and the secondary side, so the cost is more expensive, while the non-isolated LED driver is due to There is no need for a transformer, so the cost is low, but non-isolated LED drivers may cause abnormal or unstable circuits when they encounter instantaneous high voltage changes.

1 shows a conventional non-isolated linear LED driver 10, which includes a bridge rectifier 12 for rectifying an AC voltage Vac to generate a DC voltage VIN for LEDs, and an integrated circuit 14 to control the LEDs to be turned on. In the integrated circuit 14, the switches 18, 20, 22 and 24 are connected in series with the LEDs through the pins S1, S2, S3 and S4 respectively, and the controller 16 controls the switching of the switches 18, 20, 22 and 24 to determine the LEDs to be lit. . There are many situations that lead to instantaneous high voltage changes of the linear LED driver 10 , such as lightning strikes, system electrostatic discharge, multiple rapid switching of AC power or Triode Alternating Current (TRIAC) dimming, etc.

Taking TRIAC dimming as an example, FIG. 2A shows a traditional TRIAC dimmer, which includes a resistor R1 , a resistor R2 , a capacitor C1 , a triac 26 and a triac 28 , wherein the resistor R1 is a variable resistor. The triac 28 is initially off (off), so the AC voltage Vac is not input to the load, and the resistors R1 and R2 generate current according to the AC voltage Vac to charge the capacitor C1. When the voltage on the capacitor C1 reaches the bidirectional trigger diode When the breakover voltage is 26, the triac 26 is turned on and the triac 28 is turned on. When the triac 28 is turned on, the AC voltage Vac is input to the load and the capacitor C1 starts to discharge, the triac 28 will maintain the conduction state until the AC voltage is zero or the triac 28 The current I1 is less than a critical value. Simply put, the TRIAC dimmer converts the AC voltage Vac into an AC phase-cut voltage Vtr with a conduction angle. After the AC phase-cut voltage Vtr is rectified by the bridge rectifier 12 in FIG. 1 , a DC voltage VIN as shown by the waveform 30 in FIG. 2B will be generated. From the waveform 30 in FIG. 2B , it can be seen that the DC voltage VIN generated by TRIAC dimming will instantly rise from a voltage of 0V to a high voltage, resulting in an instantaneous high voltage change.

FIG. 3 is a schematic diagram of the switch 18 in FIG. 1. Since the AC voltage Vac is a high voltage, the switch 18 must be a high-voltage element. Generally, a metal-oxide-semiconductor field effect transistor (Metal-Oxide-Semiconductor Field Effect Transistor; MOSFET) or Insulated Gate Bipolar Transistor (IGBT). 4 is a voltage waveform diagram when the DC voltage VIN undergoes an instantaneous high voltage change, wherein the waveform 32 is the voltage of the pin S1 , and the waveform 34 is the voltage of the control terminal of the switch 18 . 1, 3 and 4, the input terminal 182 of the switch 18 is connected to the pin S1, the control terminal 184 of the switch 18 is connected to the controller 16, and the output terminal of the switch 18 is connected to the ground terminal. When the voltage changes, the voltage on the pin S1 rises rapidly, as shown in the waveform 32 of FIG. The voltage at the control terminal rises rapidly, as shown in the waveform 34 of FIG. 4 , when the voltage at the control terminal of the switch 18 exceeds a critical value Vth, it may cause instability and even cause the switch 18 to burn out. In some applications, some low-voltage circuits may also be connected to the output end of the switch 18. When the voltage on the pin S1 rises rapidly, a large current will be generated through the switch 18, causing the voltage on the output end of the switch 18 to rise rapidly. As a result, the low voltage circuit connected to the output end of the switch 18 cannot withstand the instantaneous high voltage change and is damaged.

US Patent Publication No. US 2010/0253245 discloses a solution to instantaneous high voltage changes, which is to add a current limiting circuit similar to an overvoltage protection circuit between the LED driver and the LED. The current limiting circuit is used to detect the voltage on the LED. voltage to control a switch in series with the LED. However, the over-current limiting circuit uses a large component that must be hung outside the integrated circuit, so it has a large parasitic capacitance, which causes the response of the current limiting circuit to be slow. In addition, the method of US Patent Publication No. US 2010/0253245 can only solve the instantaneous high voltage change caused by electric shock, and cannot solve the electrostatic discharge of the system, multiple rapid switching of AC power or Triode Alternating Current (TRIAC) ) Instantaneous high voltage changes caused by dimming, etc.

Contents of the invention

One of the objectives of the present invention is to provide a linear LED driver and its control method which can prevent circuit abnormality or instability caused by excessive transient voltage changes.

One of the objectives of the present invention is to provide a linear LED driver and a control method thereof capable of solving instantaneous high voltage changes caused by various situations.

One of the objectives of the present invention is to provide a linear LED driver and its control method that respond quickly to instantaneous high voltage changes.

According to the present invention, a linear LED driver includes a transistor, a current source and a protection circuit. The transistor has an input terminal for connecting to a light emitting diode, and when the transistor is turned on, the light emitting diode is turned on. The current source is connected to the output terminal of the transistor and used for regulating the current passing through the LED. The protection circuit is connected to the transistor, and is used to limit the maximum value of at least one of the voltage at the control terminal of the transistor and the voltage at the output terminal of the transistor, so as to prevent abnormal or unstable. Wherein the protection circuit and the transistor can be integrated in the same integrated circuit, so the parasitic capacitance of the protection circuit is small and the response is fast.

According to the present invention, a control method of a linear light emitting diode driver, the linear light emitting diode driver includes a transistor having an input terminal connected to the light emitting diode, a control terminal and an output terminal, the control method includes: turning on a transistor to Lighting the LED; and limiting the maximum value of at least one of the voltage at the control terminal of the transistor and the voltage at the output terminal of the transistor, so as to prevent the linear LED driver from being abnormal or unstable due to instantaneous voltage changes at the input terminal of the transistor.

The present invention achieves the protection of instantaneous high voltage change by limiting the maximum value of at least one of the voltage at the control terminal of the transistor and the voltage at the output terminal of the transistor, that is, this case detects the voltage at the control terminal of the transistor and the voltage at the output terminal of the transistor At least one of them, so no matter what kind of situation causes the instantaneous high voltage change, this case can definitely detect and achieve protection.

Description of drawings

Figure 1 shows a traditional non-isolated linear LED driver;

Figure 2A shows a conventional TRIAC dimmer;

FIG. 2B shows the rectified DC voltage VIN waveform of the AC phase-cutting voltage Vtr in FIG. 2A;

Fig. 3 is a schematic diagram of the switch in Fig. 1;

Figure 4 shows the voltage waveform on the switch in Figure 3 when the DC voltage VIN undergoes an instantaneous high voltage change;

Figure 5 shows a first embodiment of the present invention;

Fig. 6 shows another embodiment of the clamping circuit in Fig. 5;

Figure 7 shows a second embodiment of the present invention;

Figure 8 shows a third embodiment of the present invention;

Fig. 9 shows another embodiment of the clamping circuit in Fig. 8;

Figure 10 shows a fourth embodiment of the present invention; and

FIG. 11 shows the voltage waveform diagram when the circuit in FIG. 10 undergoes an instantaneous high voltage change.

Symbol Description:

10 Linear LED Drivers

12 bridge rectifier

14 integrated circuits

16 Controllers

18 switches

20 switches

22 switches

24 switches

26 bidirectional trigger diode

28 Three-terminal bidirectional silicon controlled switch

30 waveforms

32 waveforms

34 waveforms

36 transistors

362 input

364 console

366 output

38 transistors

382 input

384 console

386 output

40 current source

42 transistors

44 transistors

46 Protection circuit

48 clamp circuit

50 operational amplifier

52 switches

54 Zener Diodes

56 clamp circuit

58 switch

60 switches

62 operational amplifiers

64 clamp circuit

66 clamp circuit

68 Zener Diodes

70 Zener diode

72 switch

74 operational amplifier

76 switch

78 Operational Amplifiers

80 Voltage waveform of pin S1

82 waveforms

84 waveforms

detailed description

FIG. 5 shows the first embodiment of the present invention. In FIG. 5 , only the integrated circuit 14 in the linear LED driver 10 is disclosed. Please refer to FIG. 1 for the rest of the linear LED driver 10 . In the integrated circuit 14 , the input terminal 362 of the transistor 36 is connected to the LED through the pin S1 . When the transistor 36 is turned on, the LED connected in series with the transistor 36 will be turned on. The input terminal 382 of the transistor 38 is connected to the LED through the pin S2. When the transistor 38 is turned on, the LED connected in series with the transistor 38 will be turned on. Transistors 36 and 38 are high voltage elements and may be MOSFETs or IGBTs. The output terminals 366 and 386 of the transistors 36 and 38 are connected to the current source 40. The current source 40 is used to adjust the current passing through the LED to be equal to a preset current Iref, so as to control the brightness of the LED. When the sum of the current Is1 and Is2 at the output terminals of the transistors 36 and 38 is less than the current Iref of the current source 40, the voltage Vs at the output terminals of the transistors 36 and 38 drops, so that the currents Ib1 and Ib2 passing through the transistors 42 and 44 drop, causing the transistors to The voltages at the output terminals 364 and 384 of 36 and 38 rise, thereby causing currents Is1 and Is2 to rise. Conversely, when the sum of the currents Is1 and Is2 is greater than the current Iref of the current source 40, the voltage Vs rises and the currents Ib1 and Ib2 rise, causing the voltages of the output terminals 364 and 384 of the transistors 36 and 38 to drop, thereby making the currents Is1 and Is2 decline. The protection circuit 46 is connected to the transistors 36 and 38 to limit the maximum voltage on the transistors 36 and 38 to prevent the linear LED driver 10 from being abnormal or unstable due to instantaneous voltage changes of the input terminals 362 or 382 of the transistors 36 or 38 . The protection circuit 46 includes a clamping circuit 48 connected to the output terminals 366 and 386 of the transistors 36 and 38 to limit the maximum voltage of the output terminals 366 and 386 of the transistors 36 and 38 . In this embodiment, the clamping circuit 48 is an active circuit, which includes an operational amplifier 50 and a switch 52, wherein the switch 52 is connected between the output terminal of the transistor and a ground terminal, and the positive input terminal of the operational amplifier 50 The output terminals 366 and 386 of the transistors 36 and 38 are connected, the negative input terminal of the operational amplifier 50 receives a threshold value Vref1 , and the output terminal of the operational amplifier 50 is connected to the control terminal of the switch 52 . When the voltage on the pin S1 or S2 has a momentary high voltage change, the current Is1 and Is2 on the output terminals 366 and 386 of the transistors 36 and 38 rise, so that the voltage Vs of the output terminals 366 and 386 of the transistors 36 and 38 rises, and in When the voltage Vs is greater than the threshold Vref2 , the operational amplifier 50 controls the switch 52 to turn on to form a discharge path to discharge the voltage Vs, thereby limiting the maximum voltage of the output terminals 366 and 386 of the transistors 36 and 38 .

FIG. 6 shows another embodiment of the clamping circuit 48 in FIG. 5. In this embodiment, the clamping circuit 48 is a passive circuit that includes a Zener diode (Zener diode) 54 to limit the output terminals of the transistors 36 and 38. The maximum voltage of 366 and 386 , the anode of Zener diode 54 is connected to a ground terminal, and the cathode of Zener diode is connected to the output terminals 366 and 386 of transistors 36 and 38 . When the voltage on the pin S1 or S2 has an instantaneous high voltage change, the voltage Vs of the output terminals 366 and 386 of the transistors 36 and 38 rises, and when the voltage Vs is greater than a critical value (ie, the breakdown voltage of the Zener diode 54) , the Zener diode 54 is turned on to form a discharge path to discharge the voltage Vs, thereby limiting the maximum voltage of the output terminals 366 and 386 of the transistors 36 and 38 .

5 and 6, the transistors 36 and 38 share a current source 40 and a clamping circuit 48, but in other embodiments, the transistors 36 and 38 can also be configured with different current sources 40 and clamping circuits. 48.

Fig. 7 shows the second embodiment of the present invention, and it comprises transistors 36, 38, 42 and 44 and current source 40 likewise with the circuit of Fig. 5, but the protection circuit 46 of Fig. The control terminals 364 and 384 and the output terminals 366 and 386 of the transistors 36 and 38, the clamping circuit 56 detects the voltage Vs of the output terminals 366 and 386 of the transistors 36 and 38, and when the voltage Vs is greater than a critical value Vref2, the transistors 36 and 38 are turned off To limit the maximum voltage of the output terminals 366 and 386 of the transistors 36 and 38 . In the embodiment of FIG. 7 , the clamping circuit 56 is an active circuit, which includes switches 58 and 60 and an operational amplifier 62, wherein the switch 58 is connected between the control terminal 364 and the ground terminal of the transistor 36, and the switch 60 is connected between the transistor 36 and the ground terminal. Between the control terminal 384 of 38 and the ground terminal, the positive input terminal of the operational amplifier 62 is connected to the output terminals 366 and 386 of the transistors 36 and 38, the negative input terminal of the operational amplifier 62 receives a critical value Vref2, and the output terminal of the operational amplifier 62 is connected to The control terminals of switches 58 and 60. When the voltage on the pin S1 or S2 has an instantaneous high voltage change, the voltage Vs of the output terminals 366 and 386 of the transistors 36 and 38 rises, and when the voltage Vs is greater than the threshold value Vref2, the operational amplifier 62 turns on the switches 58 and 60. The transistors 36 and 38 are turned off, thereby limiting the maximum voltage at the output terminals 366 and 386 of the transistors 36 and 38 . In the embodiment of FIG. 7 , the transistors 36 and 38 share a current source 40 and an operational amplifier 62 , but in other embodiments, the transistors 36 and 38 can also be configured with different current sources 40 and operational amplifiers 62 . In addition, the clamping circuit 56 may also be a passive circuit composed of passive components.

FIG. 8 shows a third embodiment of the present invention, which includes transistors 36, 38, 42 and 44 and a current source 40 like the circuit in FIG. 5, but the protection circuit 46 in FIG. 38 control terminals 364 and 384 to limit the maximum voltage of the control terminals 364 and 384 of transistors 36 and 38 . In the embodiment of FIG. 8, both the clamping circuits 64 and 66 are passive circuits. The clamping circuit 64 includes a Zener diode 68 for limiting the maximum voltage of the control terminal 364 of the transistor 36 , the anode of the Zener diode 68 is connected to the ground terminal, and the cathode of the Zener diode is connected to the control terminal 364 of the transistor 36 . The clamping circuit 66 includes a Zener diode 70 for limiting the maximum voltage of the control terminal 384 of the transistor 38 , the anode of the Zener diode 70 is connected to the ground terminal, and the cathode of the Zener diode is connected to the control terminal 384 of the transistor 38 . When the voltages on the pins S1 and S2 have instantaneous high voltage changes, currents Icp1 and Icp2 will be generated, respectively, the parasitic capacitance Cdg1 between the input terminal 362 and the control terminal 364 of the transistor 36 and the input terminal 382 and the control terminal 384 of the transistor 38 The parasitic capacitance Cdg2 between them is charged, and the voltages Vg1 and Vg2 of the control terminals 364 and 366 of the transistors 36 and 38 rise rapidly. When the voltage Vg1 is greater than the critical value (i.e. the breakdown voltage of the Zener diode 68), the Zener diode 68 is turned on to form a discharge path for the voltage Vg1 of the control terminal 364 of the discharge transistor 36 to limit the voltage of the control terminal 364 of the transistor 36. maximum voltage. Similarly, when the voltage Vg2 is greater than the critical value (i.e. the breakdown voltage of the Zener diode 70), the Zener diode 70 is turned on to form a discharge path for the voltage Vg2 of the control terminal 384 of the discharge transistor 38 to limit the control of the transistor 38. The maximum voltage at terminal 384.

FIG. 9 shows another embodiment of the clamping circuits 64 and 66 in FIG. 8. In this embodiment, the clamping circuits 64 and 66 are active circuits. In Fig. 9, the clamping circuit 64 includes a switch 72 and an operational amplifier 74, wherein the switch 72 is connected between the control terminal 364 of the transistor 36 and the ground terminal, the positive input terminal of the operational amplifier 74 is connected to the control terminal 364 of the transistor 36, and the operational amplifier The negative input terminal of the operational amplifier 74 receives the threshold value Vref3 , and the output terminal of the operational amplifier 74 is connected to the control terminal of the switch 72 . When the voltage of the control terminal of the transistor is greater than a critical value, the switch is turned on to limit the maximum voltage of the control terminal of the transistor. The clamping circuit 66 includes a switch 76 and an operational amplifier 78, wherein the switch 76 is connected between the control terminal 384 of the transistor 38 and the ground terminal, the positive input terminal of the operational amplifier 78 is connected to the control terminal 384 of the transistor 38, and the negative input terminal of the operational amplifier 78 After receiving the threshold value Vref3 , the output terminal of the operational amplifier 78 is connected to the control terminal of the switch 76 . When the voltages on the pins S1 and S2 have instantaneous high voltage changes, currents Icp1 and Icp2 will be generated, respectively, the parasitic capacitance Cdg1 between the input terminal 362 and the control terminal 364 of the transistor 36 and the input terminal 382 and the control terminal 384 of the transistor 38 The parasitic capacitance Cdg2 between them is charged, thereby causing the voltages Vg1 and Vg2 of the control terminals 364 and 366 of the transistors 36 and 38 to rise rapidly. When the voltage Vg1 is greater than the critical value Vref3, the operational amplifier 74 turns on the switch 72 to limit the control of the transistor 36. The maximum voltage at terminal 364. Similarly, when the voltage Vg2 is greater than the threshold Vref3 , the operational amplifier 78 turns on the switch 76 to limit the maximum voltage of the control terminal 384 of the transistor 38 .

FIG. 10 shows a fourth embodiment of the present invention, which includes transistors 36 , 38 , 42 and 44 and a current source 40 like the circuit in FIG. 5 , but the protection circuit 46 in FIG. 10 includes clamping circuits 48 , 56 , 64 and 66 . The structure and operation of the clamping circuit 48 of FIG. 10 are the same as those of the clamping circuit 48 of FIG. 5 , the structure and operation of the clamping circuit 56 of FIG. 10 are the same as those of the clamping circuit 56 of FIG. It is the same as the clamping circuits 64 and 66 of FIG. 8 . In other embodiments, the clamping circuits 48 and 56 in FIG. 10 can also use passive circuits, and the clamping circuits 64 and 66 in FIG. 10 can also use active circuits.

FIG. 11 shows the voltage waveform diagram when the circuit of FIG. 10 undergoes an instantaneous high voltage change, wherein the waveform 80 is the voltage of the pin S1, the waveform 82 is the voltage Vg1 of the control terminal 364 of the transistor 36, and the waveform 84 is the output terminal 366 of the transistor 36. The voltage Vs. Referring to Figure 10 and Figure 11, when an instantaneous high voltage change causes the voltage of the pin S1 to rise rapidly, as shown at time t1 in Figure 11, the voltages Vg1 and Vs both start to rise, and at time t2, the voltage Vg1 has reached the clamp The breakdown voltage of the Zener diode 68 in the circuit 64, so the Zener diode 68 is turned on to limit the maximum value of the voltage Vg1, so as to prevent the linear LED driver from being unstable or burnt out. At this time, the voltage Vs still continues to rise. When the voltage Vs is greater than the threshold value Vref1, the switch 52 of the clamping circuit 48 is turned on to discharge the voltage Vs, but because the voltage of the pin S1 is still changing sharply, the output terminal 366 of the transistor 36 continues to generate a large current Is1, The clamping circuit 48 cannot completely discharge the current Is1 to the ground terminal, so the voltage Vs still continues to rise. When the voltage Vs reaches the critical value Vref2, the switch 58 in the clamping circuit 56 is turned on so that the transistor 36 is turned off, so the output terminal 366 of the transistor 36 no longer outputs the current Is1, and as the clamping circuit 48 continues to discharge, the voltage Vs begins to drop . When the voltage Vs is lower than the critical value Vref2, the switch 58 in the clamping circuit 56 is closed, so the voltage Vg1 of the control terminal 364 of the transistor 36 starts to rise, and the voltage Vg1 is not enough to turn on the transistor 36 at this time, so the voltage Vs still continues decline. After the transistor 36 is turned on again, if the voltage of the pin S1 is still changing sharply, as shown at time t4, the voltage Vs will rise again. Afterwards, the aforementioned operations are repeated until the voltage of the pin S1 is stabilized, and the voltage Vs will be stabilized within a normal operating range.

Claims (28)

1. a linear LED drive, it is characterised in that including:

One transistor, has an input for connecting luminous diode, wherein when this transistor turns, this luminescence two Pole pipe is lit；

One current source, connects the outfan of this transistor, in order to regulate the electric current by this light emitting diode；And

One protection circuit, connects this transistor, in order to limit the voltage controlling end of this transistor and the defeated of this transistor Go out the maximum of voltage at least one of which of end, to avoid defeated because of this transistor of this linear LED drive Enter the transient voltage change of end and occur abnormal or unstable.

2. linear LED drive as claimed in claim 1, it is characterised in that this protection circuit includes One clamped circuit connects the outfan of this transistor, to limit the maximum voltage of the outfan of this transistor.

3. linear LED drive as claimed in claim 2, it is characterised in that this clamped circuit includes:

One switch, is connected between outfan and an earth terminal of this transistor；And

One operational amplifier, connects outfan and this switch of this transistor, big at the voltage of the outfan of this transistor When a marginal value, turn on this switch to limit the maximum voltage of the outfan of this transistor.

4. linear LED drive as claimed in claim 2, it is characterised in that this clamped circuit includes One zener diode is in order to limit the maximum voltage of the outfan of this transistor, and the anode of this zener diode connects one and connects Ground end, the negative electrode of this zener diode connects the outfan of this transistor.

5. linear LED drive as claimed in claim 1, it is characterised in that this protection circuit includes One clamped circuit connects control end and the outfan of this transistor of this transistor, detects the electricity of the outfan of this transistor Pressure, when the voltage of the outfan of this transistor is more than a marginal value, closes this transistor to limit the defeated of this transistor Go out the maximum voltage of end.

6. linear LED drive as claimed in claim 5, it is characterised in that this clamped circuit includes:

One switch, is connected between control end and an earth terminal of this transistor；And

One operational amplifier, connects outfan and this switch of this transistor, big at the voltage of the outfan of this transistor When this marginal value, turn on this switch to close this transistor.

7. linear LED drive as claimed in claim 1, it is characterised in that this protection circuit includes One clamped circuit connects the control end of this transistor, to limit the maximum voltage controlling end of this transistor.

8. linear LED drive as claimed in claim 7, it is characterised in that this clamped circuit includes One zener diode is in order to limit the maximum voltage controlling end of this transistor, and the anode of this zener diode connects one and connects Ground end, the negative electrode of this zener diode connects the control end of this transistor.

9. linear LED drive as claimed in claim 7, it is characterised in that this clamped circuit includes:

One switch, is connected between control end and an earth terminal of this transistor；And

One operational amplifier, connects control end and this switch of this transistor, big at the voltage controlling end of this transistor When a marginal value, turn on this switch to limit the maximum voltage controlling end of this transistor.

10. linear LED drive as claimed in claim 1, it is characterised in that this protection circuit includes:

One first clamped circuit, connects the outfan of this transistor, to limit the maximum voltage of the outfan of this transistor；

One second clamped circuit, connect this transistor controls end and the outfan of this transistor, detects this transistor The voltage of outfan, when the voltage of the outfan of this transistor is more than a marginal value, closing this transistor should to limit The maximum voltage of the outfan of transistor；And

One the 3rd clamped circuit, connects the control end of this transistor, to limit the maximum voltage controlling end of this transistor.

11. linear LED drive as claimed in claim 10, it is characterised in that this first clamped circuit Including:

One switch, is connected between outfan and an earth terminal of this transistor；And

One operational amplifier, connects outfan and this switch of this transistor, big at the voltage of the outfan of this transistor When second marginal value, turn on this switch to limit the maximum voltage of the outfan of this transistor.

12. linear LED drive as claimed in claim 10, it is characterised in that this first clamped circuit Including a zener diode in order to limit the maximum voltage of the outfan of this transistor, the anode of this zener diode connects One earth terminal, the negative electrode of this zener diode connects the outfan of this transistor.

13. linear LED drive as claimed in claim 10, it is characterised in that this second clamped circuit Including:

One switch, is connected between control end and an earth terminal of this transistor；And

One operational amplifier, connects outfan and this switch of this transistor, big at the voltage of the outfan of this transistor When this marginal value, turn on this switch to close this transistor.

14. linear LED drive as claimed in claim 10, it is characterised in that the 3rd clamped circuit Including a zener diode in order to limit the maximum voltage controlling end of this transistor, the anode of this zener diode connects One earth terminal, the negative electrode of this zener diode connects the control end of this transistor.

15. linear LED drive as claimed in claim 10, it is characterised in that the 3rd clamped circuit Including:

One switch, is connected between control end and an earth terminal of this transistor；And

One operational amplifier, connects control end and this switch of this transistor, big at the voltage controlling end of this transistor When a marginal value, turn on this switch to limit the maximum voltage controlling end of this transistor.

16. linear LED drive as claimed in claim 1, it is characterised in that this transistor is high pressure Element.

17. linear LED drive as claimed in claim 1, it is characterised in that this transistor is metal Oxide semiconductor field effect transistor.

18. linear LED drive as claimed in claim 1, it is characterised in that this transistor is insulation Grid bipolar transistor.

The control method of 19. 1 kinds of linear LED drive, it is characterised in that this linear light emitting diode drives Dynamic device comprises a transistor and has an input being connected with light emitting diode, control end and an outfan, this control Method comprises the following steps:

Turn on this transistor to light this light emitting diode；And

Limit the maximum of the voltage at least one of which of the voltage controlling end of this transistor and the outfan of this transistor Value, to avoid this linear LED drive, because of the change of the transient voltage of the input of this transistor, exception occurs Or it is unstable.

20. control methods as claimed in claim 19, it is characterised in that it is maximum that this limits the voltage on this transistor The step of value includes the maximum voltage limiting the outfan of this transistor.

21. control methods as claimed in claim 20, it is characterised in that this limits the outfan of this transistor When the step of big voltage is included in the voltage of the outfan of this transistor more than a marginal value, form a discharge path for putting The voltage of the outfan of this transistor of electricity, to limit the maximum voltage of the outfan of this transistor.

22. control methods as claimed in claim 20, it is characterised in that this limits the outfan of this transistor When the step of big voltage is included in the voltage of the outfan of this transistor more than a marginal value, close this transistor to limit The maximum voltage of the outfan of this transistor.

23. control methods as claimed in claim 19, it is characterised in that it is maximum that this limits the voltage on this transistor The step of value includes the maximum voltage controlling end limiting this transistor.

24. control methods as claimed in claim 23, it is characterised in that what this limited this transistor controls end When the step of big voltage is included in the voltage controlling end of this transistor more than a marginal value, form a discharge path for putting The voltage controlling end of this transistor of electricity, to limit the maximum voltage controlling end of this transistor.

25. control methods as claimed in claim 19, it is characterised in that it is maximum that this limits the voltage on this transistor The step of value includes:

Limit the maximum voltage of the outfan of this transistor；And

Limit the maximum voltage controlling end of this transistor.

26. control methods as claimed in claim 25, it is characterised in that this limits the outfan of this transistor When the step of big voltage is included in the voltage of the outfan of this transistor more than a marginal value, form a discharge path for putting The voltage of the outfan of this transistor of electricity, to limit the maximum voltage of the outfan of this transistor.

27. control methods as claimed in claim 25, it is characterised in that this limits the outfan of this transistor When the step of big voltage is included in the voltage of the outfan of this transistor more than a marginal value, close this transistor to limit The maximum voltage of the outfan of this transistor.

28. control methods as claimed in claim 25, it is characterised in that what this limited this transistor controls end When the step of big voltage is included in the voltage controlling end of this transistor more than a marginal value, form a discharge path for putting The voltage controlling end of this transistor of electricity, to limit the maximum voltage controlling end of this transistor.